<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    body{
      margin: 8px;
    }
    div{
      width: 300px;
      background-color:rgba(0,0,0,0.3);
      padding: 10px;
      position: relative;
    }
    input{
      width: 260px;
      padding: 10px 20px;
      border: none;
    }
    img{
      position: absolute;
      top: 14px;
      right: 40px;
      width: 28px;
      height: 28px;
    }
    p{
      margin: 0;
      font-size: 12px;
      color: red;
    }
  </style>
</head>
<body>
<div>
  <input type="text" placeholder="请输入用户名">
  <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
  <p>用户名不能为空!</p>
</div>
</body>
</html>